feat: Add Graceful Plugin Shutdown to Runner

This change introduces a shutdown lifecycle hook for plugins. The `PluginManager` now has an `async def shutdown()` method that will call `await plugin.shutdown()` on any registered plugins that implement the method. This is called from `Runner.close()`, allowing plugins to perform cleanup tasks like flushing logs or closing connections when the runner instance is being closed. This improves the reliability of plugins that perform background operations.

PiperOrigin-RevId: 831037737

src/google/adk/plugins/base_plugin.py

@@ -187,6 +187,14 @@ class BasePlugin(ABC):
     """
     pass
 
+  async def close(self) -> None:
+    """Method executed when the runner is closed.
+
+    This method is used for cleanup tasks such as closing network connections
+    or releasing resources.
+    """
+    pass
+
   async def before_agent_callback(
       self, *, agent: BaseAgent, callback_context: CallbackContext
   ) -> Optional[types.Content]: